Literature Review

Table of Contents

Review of Literature

What is wind?

Wind is air in motion, caused by the uneven heating of the Earth by the sun. Wind occurs when warm air rises, and cooler air moves in to fill the space. It is estimated that 2% of the solar energy reaching the earth is converted into wind energy. Air is constantly being interchanged between the warm tropics and the cold polar caps. The rotation of the Earth also produces wind.

The sun radiates the most heat over the equator and therefore the air there is warmer. Air from both hemispheres is constantly moving toward the equator. The rotation of the Earth causes the cool winds to be deflected from east to west. As the surface of the earth heats and cools unevenly, pressure zones are created that make air move from high pressure to low pressure areas.

What is wind energy?

The process by which the kinetic energy of wind is used to generate mechanical power or electrical energy is known as wind power or wind energy. Kinetic means being related to or produced by motion such as the blowing wind.

A windmill converts the force of the wind into a turning force acting on the rotor blades. The strength of this turning force is known as torque.

Wind speed and energy:

The amount of energy that can be captured from the wind is exponentially proportional to the speed of the wind. If a windmill were perfectly efficient, the power generated is approximately equal to:

P (watts) = 1/2 D (air density) x A (area of rotor) x V cubed (wind velocity)
Air density at sea level and 14 degrees C = 1.225.

Therefore, if wind speed is doubled, the power in the wind increases by a factor of eight, i.e. 2 x 2 x 2. In reality, because wind turbines are not perfectly efficient, changes in wind velocity do not have such a dramatic effect on wind power. Betz' Law states that you can only convert approximately 59 % of the wind energy to mechanical energy using a wind turbine. However, small changes in velocity do impact on available energy, making wind speed an important factor to consider in the placement of a wind turbine.

The chart below illustrates that a doubling of wind velocity increases power available by a factor of eight.

History of Wind Power:

Wind has been used for centuries to propel ships and the wind routes were well known and used by explorers such as Magellan and Columbus.

Wind power was used as a source of mechanical energy on land for thousands of years. The Babylonians constructed windmills for irrigation as early as 1700 BC and Europeans were using windmills by 1000 AD.

The Dutch used windmills to drain the land and used eight basic types. Dutch settlers introduced windmills to the United States in the early 1600s.

Daniel Halliday invented a new style of windmill, which many believe encouraged the rapid settling of the American West. More than 6.5 million windmills were sold in the US between 1880 and 1935. They were used to pump water, grind grain and cut lumber. Some small electrical generating systems were used to produce direct current by 1900. Cheap electricity was introduced in th 1940s and most of the wind powered generating systems in rural areas were considered obsolete and fell into disuse.

Wind turbine is the name given to a complete, electricity generating windmill. In its simplest form, it consists of a tower, blades, generator and, if electricity is to be stored, batteries. There are large windfarms in many areas of the world.

Wind Turbine Rotor Design:
There has been a great deal of research on rotor design including whether the turbine will be upwind (rotor facing the wind) or downwind (rotor on the lee side of the tower), the number, size and shape of blades, the load (forces acting on the rotor in high wind) and other rotor aerodynamic considerations.

Generally speaking, larger windmill rotors and higher wind speed, produce more power. The old Western windmills had many, wide blades. During very high winds, they were exposed to extremely high forces known as loads and were often damaged. Modern wind turbines by law, have to be able to withstand extreme winds that may only occur once every 50 years.

Most wind turbines are the classic Danish three-bladed design with the rotor positioned up-wind (facing the wind). Even numbers of blades cause instability. Some designs are two bladed, saving the cost of a blade and reducing rotor weight. They need higher rotational speeds to produce the same amount of power as a three bladed design. These speeds produce more noise. There are one bladed designs that require a counter-balance on the other side of the hub. They also require higher rotational speed.

Aerodynamics of Rotors:
Rotor blades act like airfoils. An airfoil is a structure around which air flows creating lift. Rotor blades have a special shape so that when the wind passes over them, it moves faster over one side. Bernoulli's Principle states that increased air velocity produces decreased pressure.

When the wind blows there is a pocket of low pressure formed on the downwind side of the blade. The blade is pulled toward the low pressure making the rotor turn. This is called lift. The lift force is stronger than the force, known as drag, acting on the front side of the blade. The combination of lift and drag causes the rotor to spin like a propeller, and the turning shaft spins a generator to make electricity. In wind turbine design, the objective is to have a high lift-to-drag ratio. This is accomplished by twisting the blades. The blades are twisted so that the wind hits them at the correct angle of attack. This twist is known a pitch.

Experimental Design